1. Field of the Invention
The present invention relates to a process for forming functional films, particularly deposition useful for various purposes, specially for semiconductor devices and electrophotographic photosensitive devices.
2. Description of the Prior Art
A number of possible processes have been disclosed up to now for forming deposition films. For instance, for producing tilms of amorphous silicon deposit, there have been tried the vacuum deposition process, plasma CVD process, CVD process, reactive sputtering process, ion plating process and photo-CVD process, etc. Generally, the plasma CVD process is industrialized and widely used for this purpose.
However, deposition films of amorphous silicon still admit of improvements in overall characteristic including electric and optical properties, various characteristics to fatigue due to repeated uses or to environmental use conditions, and additionally productivity including product uniformity, reproducibility, and mass-producivity.
The conventional plasma CVD process, as compared with the conventional CVD process, is complicated in the reaction process to deposit amorphous silicon and involves many unknown things in the reaction mechanism. The film formation in the plasma CVD process is affected by a number of parameters (e.g. substrate temperature, flow rates and mixing ratio of feed gases, pressure during film formation, high-frequency power used, electrode structure, structure of deposition chamber, rate of evacuation, and plasma generation method). These various parameters, combined with one another, cause sometimes an unstable plasma, which exerts marked adverse effects on the deposited film. In addition, the parameters characteristic of the deposition apparatus must be determined according to the given apparatus, so that it is difficult in practice to generalize the production conditions.
The conventional plasma CVD process is regarded at the present time as the best method for the purpose of obtaining amorphous silicon films which have such electrical and optical properties as to fulfill various application purposes.
However, it is necessary in certain applications of the deposition film to realize reproducible mass production thereof while satisfying needs for the film of a large area, uniform thickness, and uniform quality. In consequence, much investment is required for the mass production equipment, the administration items for the mass production become complicated, tolerances in quality control become narrow, and the regulation and adjustment of equipment becomes delicate. These matters in the production of amorphous silicon deposit films by the plasma CVD process are pointed out as the problems to be solved in the future.
On the other hand, the conventional CVD process requires a high operational temperature and has not given the deposition film having such characteristics that the film can be used practically.
Thus there is an intensive need for a process for producing amorphous silicon films which, as stated above, realizes mass production thereof at a low equipment cost while securing practically acceptable characteristics and uniformity of the films.
The above stated matters also apply to other functional films, e.g. silicon nitride, silicon carbide, and silicon oxide films.